Tire condition monitor

Abstract

A tire condition sensing circuit includes a ferrite element that changes from a ferromagnetic to a nonferromagnetic state in response to a temperature increase above the material Curie point. The change in state of the element affects the sensor circuit operation such that a normal inductive coupling effect is altered and an abnormal tire condition indicated.

I claim: 
     
       1. A condition sensing circuit adapted for mounting on a vehicle wheel to monitor the condition of a tire and provide an indication of a change in condition through inductive coupling with signal monitoring and indicating apparatus mounted on the vehicle chassis, the sensor circuit comprising: A. a toroidal ferrite element that switches from a ferromagnetic state to a nonferromagnetic state when the element temperature exceeds the Curie point established for the ferrite material;   B. an inductive circuit for establishing a particular inductive coupling effect indicative of a normal tire condition when in proximate position to the signal monitoring apparatus; and   C. means to couple the ferrite element into the inductive circuit comprising a pair of inductive coils wound on opposite halves of the toroid element and forming parallel branch circuits such that current flow in one of the branches is reversed when the element switches to a nonferromagnetic state due to a temperature above the Curie point, said current reversal effectively altering the particular inductive coupling effect for an abnormal tire condition indication.   
     
     
       2. The condition sensing circuit as set forth in claim 1 wherein a pressure sensing switch is connected into the means coupling the ferrite element into the inductive circuit such that either an open-circuited pressure switch or a nonferromagnetic element alters the inductive coupling effect indicative of a normal tire condition. 
     
     
       3. In a vehicle tire condition monitoring system including chassis mounted signal monitoring and indicating circuits, the monitoring circuit responsive to a magnetic field reversing effect to provide a modulation signal to the indicator circuit indicative of a normal tire condition, a wheel mounted condition sensing circuit comprising: A. a toroidal ferrite element exhibiting a change-of-state characteristic from a ferromagnetic to a nonferromagnetic in response to a temperature above the ferrite material Curie point;   B. a first branch circuit comprising a first pair of series connected inductive coils, one of the pair being wound on a first half portion of the toroid while the other of the pair is mounted to affect the monitoring circuit when positioned relative thereto;   C. a second branch circuit comprising a second pair of series connected inductive coils, one of the pair being wound on a second half portion of the toroid while the other of the pair is mounted to affect the monitoring circuit when positioned relative thereto;   D. a coupling circuit interconnecting the branch circuits by a pair of node connections, the first of the nodes interconnecting the inductive coils that are wound on separate halve portions of the toroid while the second of the nodes interconnects the inductive coils that are positioned relative to the monitoring circuit, said coils being effective to provide a reverse magnetic field coupling effect when the ferrite element is in its ferromagnetic state but are ineffective to provide such reverse coupling effect when the ferrite element is in a nonferromagnetic state, said ineffectiveness establishing an abnormal temperature condition indication by reason of the absence of a modulation signal being generated in the monitoring circuit.   
     
     
       4. The tire condition monitoring system as set forth in claim 3 wherein the coupling circuit includes a switch in a normally close-circuited condition such that either an open-circuited switch or a nonferromagnetic ferrite element establish an ineffective reverse coupling affect and an abnormal tire condition is recognized. 
     
     
       5. The tire condition monitoring system as set forth in claim 4 wherein the coupling circuit further includes an inductor connected in series with the switch to prevent excessive loading of the signal monitoring circuit when the switch is closed and the ferrite element is in its ferromagnetic state.